Process for the prevention of build-up of deposits in vapor conduits



Sept. 23, 1969 H. ROTHERT 3,468,849

PROCESS FOR THE PREVENTION OF BUILD-UP OF DEPOSITS IN VAPOR CONDUITSFiled Nov. 8. 1966 INVENTOR. Hoesr POTHE/ZT ATTORNEYS.

United States Patent US. Cl. 26075 3 Claims ABSTRACT OF THE DISCLOSUREMethod for preventing build-up of poly(ethylene terepthalate) polymerdeposits in vapour conduits comprising injecting a jet of vaporizedglycol into a stream of vapor containing said polymer material.

The invention relates to a process for the operation of a jet pump forthe conveyance of a substance which has a tendency to form annoyingprecipitates in a vacuum that are soluble in a non-aqueous solvent. Aspray condenser is made use of when the substance to be conveyed is avaporous medium that becomes precipitated, deposited or condensed in thespray condenser. Through this means there is formed a vacuum throughwhich the medium is drawn, and thus conveyed onward. A jet pump isprovided to supplement the condenser, and it serves the purpose ofmaintaining the vacuum in the spray condenser. The process, however, isalso suitable for operating an installation that comprises only the jetpump, without a condenser. Such equipment for conveying the substance isof simple design, without rotating parts, and therefore it makespossible easy control of all parts of the equipment which come intocontact with the substance to be conveyed. This advantage is made use ofby the chemical industry in particular.

Difiiculties arise, however, when the substance which is to be conveyeddisplays in a vacuum a tendency to form annoying precipitates thatcannot be removed by water or steam. This is, for example, the case whenthe substance to be conveyed is a condensable vapor which containsconstituents, impurities, for example, in the form of dust, or that canbe condensed or can be Sublimated. Examples of this are impuritieswhich, in the region of moisture saturation, form hydrates. This appliesin par ticular for calcium, which forms cement-like deposits in theequipment, particularly in the jet tube of the jet pump and in thepiping of the equipment. The alkylene glycol propellants hereof dissolvesuch deposits. Another example of impurities causing disturbances arethe alkali deposits of substances which in vacuum-metallurgy have to beconveyed in vapor form. In the zone of the high temperatures for meltingmetals, these alkali deposits evaporate, and they then depositthemselves in the cooler regions of the piping.

Disturbing deposits can also be formed from dust, and can cause erosionsin the apparatus, particularly in the bends of piping.

In chemical equipment, the substance conveyed forms annoying depositsmainly during a change in its partial pressure and in its concentrationin the vacuum stage of the equipment.

The fundamental problem solved by the invention is to prevent theformation of such deposits, or precipitates, and particularlycement-like deposits in the jet tube of the jet pump used in conjunctionwith a poly(ethylene terephthalate) reactor. The invention can be madeuse of in those cases where the precipitates are soluble in a nonlCCaqueous, alkylene glycol, or one including a cycloaliphatic ring.

In accordance with the invention, this problem is solved in that thevapor jet of the jet pump is a vapor stream of an alkylene glycol per seforceably introduced into the system. The result of this is that thissolvent prevents a formation of annoying deposits because it dissolvesand washes away the condensing or sublimating precipitates directly asthey are formed.

The process according to the invention can, for example, be applied toequipment wherein the substance to be conveyed is ethylene glycol, whichis sucked continuously out of a reactor vessel in which monomericbis-(2-hydroxyethyl) terephthalate is heated and thereby becomespolymerized, being sucked continuously out of the reactor by a vacuummaintained by a jet pump.

Such a process is described in application Ser. No. 519,836 filed Jan.11, 1966. If in the way that is obvious from the prior art, the pumpingjet of the jet pump is formed of steam, then, under the influence of thecooler surfaces of the jet pump there is formed a precipitate whichconsists of the volatile polymers or condensation products and whichclings tightly to the walls of the equipment in the form of smears andgels. This deposit alters the flow cross sections, and is, therefore, agreat disturbance. It also makes it more diflicult, by admixture ofwater to the ethylene glycol, to reuse the glycol. The mixture of waterand ethylene glycol cannot without further steps be separated out aswaste, and, therefore, contaminates the Water flowing off.

All these problems are avoided when, in accordance with the invention,the pump jet is formed of vaporized ethylene glycol with or withoutdiluents and in which the undesirable deposits dissolve.

The glycol jet that enters the jet pump as a jet of vapor, is formed ofthe same substance that is conveyed by the jet pump; that is, with thepreferred example of carrying out the invention, of ethylene glycol. Itis, however, also possible to use aliphatic glycols other than ethyleneglycol for these jets. Such materials provide a non-aqueous, preferablyneutral propellant medium for the vacuum system. Specific examples ofsuch glycols include 1,4-dimethylolcyclohexane, butylene glycol,hexarnethylene glycol, or glycols having side chains, for example,1,2-propylene glycol or the aforementioned dimethylolcyclohexane.

The invention also concerns equipment for carrying out the new process.This equipment is characterized in that, between the suction connectionsof the jet pump and the reactor vessel is interposed thealready-mentioned spray condenser.

The vacuum vapor-jet pump can preferably be heated, because cold pumpwalls would promote a formation of the described deposits.

The accompanying drawing shows a preferred example of construction ofthe equipment for carrying out the process of the invention.

Into an upright reactor 1 is introduced a monomeric bis-(2-hydroxyethyl)terephthalate, and by means of vanes on a vertical rotating shaft, theseelements not being illustrated, it is spread in a thin layer over theinner wall of the reactor 1, out of which layer the ethylene glycolbecoming liberated is vaporized under the influence of the heating ofthe reactor by its hot-jacket 2 and under the influence of a vacuum.These vapors are conveyed out of the reactor 1. This objective isachieved by a spraycondenser 4 and a jet pump 5.

The outlet connectors 3 of the reactor 1 lead to a cylindrical head part6 of the spray condenser 4. This head part 6 is provided with a heatingjacket 7. Into this head 6 there flow the glycol vapors conveyed throughthe outlet connections 3, going then downward. In this head part 6 isdisposed a rotating shaft 8 carrying a worm 8a that serves for cleaningthe inner walls of the head part 6. The head part 6 is open at itsbottom, and merges into an inner condenser tube 9, which runs downwardand is likewise open at its bottom, and is surrounded by an outer tube10 of greater diameter. This outer tube runs farther down than the innertube, and at its bottom runs conically into an outlet opening, to whichis connected a barometric outlet tube that is not shown in detail. Thetube 9 is closely above the outer tube 10 provided with a ring ofspraynozzles 12. Another ring of nozzles 13 is disposed in the wall ofthe tube 10, closely above the lower end of the tube 9. The nozzles 12and 13 are supplied with a cooling fluid, Whose purpose is to condensethe glycol vapors. A glycol, preferably ethylene glycol, serves as thecooling fluid, although other spray media may be used if desired.Instead of a spray condenser as exemplified herein, other condensingmeans, e.g. surface condenser means, may be used.

The tube 10 of the spray-condenser has close below its upper end anoutlet 14, which is connected to the head 15 of the jet pump 5. Jet pumpis provided with a heating jacket 16, which extends round the head 15and the jet venturi 17. At the top of the head 15 is inserted a jetnozzle 18 directed downward in the axial direction, and it supplies thepumping jet of glycol vapor.

The jet pump 5 maintains the vacuum in the spraycondenser 4 and in thereactor 1. The vapors leaving the jet pump are conveyed into amixed-condensation tank 19. Unprecipitated vapors or gases emerge froman outlet 21; while the liquid ethylene glycol is drawn off from anopening 22 provided at the lower end of the tank 19. This opening isconveniently provided with a barometric leg (not shown).

The more or less polymerized substance coming out of the bottom of thereactor 1 can be introduced into a second similar reaction vessel, whichforms a second polymerization stage. The product obtained in this waycan then be introduced in a third stage into a reactor vessel,corresponding to the one that is illustrated. In this case it isdesirable to make the vacuum in the first stage 40 mm. Hg pressure, inthe second stage 5 to 30 mm. Hg pressure, and in the third stage 0.1 mm.Hg pressure. When this is done, then there are obtained for the firststage the following operating conditions: a quantity of ethylene glycolamounting to 88 kg. hourly is liberated. In the spraycondenser 4, 30 kg.hourly of this is separated out. The mixed condenser 19 has a diameterof about 300 mm. and a height of about 1000 mm. Its spray nozzle is fedwith 660 kg. of glycol per hour. The jet pump 9 conveys to it a quantityof ethylene glycol vapor amounting to 58 kg. per hour. To outlet 21there can be connected a conventional steamjet vacuum pump, which sucksoff the inert gases, and serves the purpose of producing a vacuum in theequipment before starting operations, and which then maintains thisvacuum. The water fed to the nozzle of such a steam-jet pump has atemperature of 35 C. and the steam fed to the pump has a pressure of 5atmospheres gage. The consumption of steam amounts to 20 kg. per hour,and the consumption of water amounts to 250 liters per hour.

In the second stage, 7.8 kg. per hour of ethylene glycol is liberated inthe reactor 1. Of this, 2.7 kg. per hour is condensed in thespray-condenser. In the mixed condenser 19 the amount of ethylene glycolsprayed in amounts to 132 kg. per hour. The dimensions of this condenser19 are the same as in the case of the first stage. In it 5.1 kg. ofresidual ethylene glycol is condensed hourly. The steam- .jet pumpconnected on consumes 35 kg. per hour of steam at 5 atmospheres gagepressure, and consumes 1.2 cubic meters per hour of Water at 35 C.

In the reactor 1 of the third stage, 1 kg. per hour of ethylene glycolis liberated. The vacuum in this reactor amounts to 0.1 mm. Hg pressure.There is also sucked out of the reactor 1, 1 kg. per hour of inert gas.In the spraycondenser 4, 0.66 kg. per hour of ethylene glycol iscondensed. The vapor-jet pump 5 is fed with 25 kg. per hour of glycolvapor hourly, at 1.05 atmospheres gage pressure. The steam-jet vacuumpump connected to the condenser 19 consumes 50 kg. per hour of steam at5 atmospheres gage pressure, and 2 cubic meters per hour of water at 30C.; while the spray nozzle of the mixed condenser 19 consumes 550 kg.per hour of glycol at 28 C.

For the purpose of condensing 2 kg. per hour of ethylene glycol vapor,plus about 1 kg. per hour of inert gas, from an 0.1 mm. Hg pressure toatmospheric pressure, there are needed:

When using the conventional When using the process claimed steam-jetsuction elements by the invention Ethylene glycol vapor as the meansSteam at 5 atmospheres gage as to operate the jet-suction elements themeans for multistage jetat a pressure of 1.05 atmosphere suctionelements: 275 kg. per

gage: 38 kg. per hour. hour. Ethylene glycol fluid at 28 C. for Coolingwater at 30 C.: 30.25 m} per hour.

This comparison shows that the process according to the invention is ofgreat economy.

A further advantage is obtained in that it is now possible to eliminatedisturbing waste-water problems. Furthermore, the mixed condenseroperated with ethylene glycol condenses and recovers ethylene glycolescaping from the reaction, so that it is merely necessary, in startingup the equipment, to put in the required amount of ethylene glycol.

What I claim is:

1. In a process for producing poly(ethylene terephthalate), a method forpreventing the build-up of undesired deposits of the polymer in vaporconduits which comprise:

(a) moving up at subatmospheric pressure in a conduit a stream of vaporcontaining said polymer material which has a tendency to deposit on thewalls of said conduit;

(b) forceably injecting into said stream of vapor a jet of vaporizednon-aqueous, alkylene glycol suificient to reduce the pressure in thesystem, and

(c) removing the mixture of said glycol and said polymer from saidconduit.

2. A process in accordance with claim 1 in which the alkylene glycol isethylene glycol.

3. A process in accordance with claim 1 in which the alkylene glycol is1,4-dimethylolcyclohexane.

References Cited UNITED STATES PATENTS 3,315,879 4/1967 Jennings 230453,376,353 4/1968 Tate 260-637 2,793,235 5/1957 Jenkinson 260-637 WILLIAMH. SHORT, Primary Examiner L. P. QUAST, Assistant Examiner US. Cl. X.R.

